Three boxes: engine, cabin, boot. The third of them is the definition. A sedan’s rear window is fixed to the body and stays put when the boot lid rises, and a parcel shelf divides the load bay from the passengers. That single hinge line is the entire difference between a sedan and a liftback of identical outline, which is why the two get confused so reliably.
The sealed boot is worth something. It keeps road noise out of the cabin, adds structure across the back of the car, and hides whatever is in it. What it costs is the ability to load anything taller than the aperture. Sedans stretch across most of the market: the Toyota Camry at 4,885 mm, the Tesla Model 3 at 4,720 mm, the Xpeng P7 at 5,017 mm. Electric platforms have suited the shape, since a flat floor and no transmission tunnel return cabin space to the body without any change to the silhouette.